BLANKET BOG
DESCRIPTION
The word ‘corog’ could be a form of the word ‘corsog’ meaning ‘boggy’. If that is the case, the name of our upland area Bwlch Corog means ‘boggy pass’ which describes the landscape very well.
Bogs are areas of waterlogged peat soil formed in climates of high rainfall and low evapotranspiration. The peat is dead plant material, mainly sphagnum mosses. The waterlogged acidic conditions prevent decomposition. There are two types of bog: raised bog, found generally in lowland areas in Britain; and blanket bog, associated with the uplands, which is the type present at Bwlch Corog.
Raised bogs are natural formations originating from lakes forming in depressions in the landscape. Dead plant material on the lake beds create peat which over thousands of years builds up until the lake is filled in. The peat continues to form so that the bogs eventually form a dome slightly higher than the surrounding land – hence the name ‘raised bog’.
Blanket bogs are not natural in Wales, being artefacts of past human impacts on the landscape. Some blanket bogs are naturally formed, for example in Northern Scotland and Scandinavia. After the Ice Age, native forest of birch, pine and other trees colonised the landscape, including upland areas. The forest was cleared for agriculture about 4,500 years ago. In areas of high rainfall, rain leached out the nutrients from the soil now that it was unprotected by tree cover.
This caused the soil to become more acidic and the leached minerals were deposited in a ‘hard pan’ at a lower depth in the soil. In some places, the charcoal from the burnt trees created the hard pan. The hard pan impeads drainage and causes the land to become waterlogged. With cultivation no longer possible on this land, the acidic, infertile soils were colonised by heathers and mosses, which form the peat.
By around 1,000 years ago, the peatlands were well established, growing in depth at about 1 millimeter per year. Some peatlands are associated with heather: upland heath. The more waterlogged peatlands are associated with sphagnum mosses: the name ‘blanket’ bog comes from their homogenous appearance and the way they form over large expanses of undulating ground. The bogs are characterised by pools of open water and extensive patches of sphagnum moss that appear to be floating on water, so that the ground moves under your feet for a wide area when walking on it.
BWLCH COROG
At Bwlch Corog, the upland area of around 120 hectares (300 acres) was drained during the 1930s by digging 11 kilometers of drainage ditches or ‘grips’. These were installed on the flatter areas of blanket bog as well as on the heathland slopes. The idea was to dry the soil to make productive pasture. We have no record of how the vegetation developed after drainage but by 2017, when Bwlch Corog was acquired for the project, there was a well established monoculture of purple moor grass (Molinia caerulea) covering most of the area.
MOLINIA
The tendency for Molinia to dominate moorland vegetation is a widespread phenomenon in Britain and is thought to be linked to drainage, burning, increase in sheep grazing in preference to cattle, and nitrogen deposition from air pollution.
Molinia is a native grass species and a significant component of upland heath and some other special habitats, for example Rhos pasture, certain wet woodlands, and bog woodlands.
The name purple moor grass comes from the colour of the early spring growth – during the summer it is green. It is unpalatable for many grazing animals and is a deciduous grass, meaning that the leaves (blades) die off in the winter. This creates the enigmatic straw colour of the winter moorland. The problem with Molinia is that by not being grazed it outcompetes other plants, and the dead grass forms a dense mat which smothers the ground, preventing other plants from growing. It ends up being a monoculture with little diversity or wildlife interest, which is exactly what we have at Bwlch Corog. It has a characteristic structure of large tussocks with minimal space between tussocks, which can make moorland walking a torment.
DRAINAGE
There are other problems associated with drainage of the bogs. A bog in good condition holds a huge volume of water and releases it slowly into upland streams. This is particularly important in the areas where bogs occur, where rainfall can be heavy and prolonged. If the bog is drained, rain water is taken off the hill almost immediately through the grips and into the streams and rivers. Floods are therefore more frequent and more severe downstream and these often affect populated areas. Even without flooding, the excessive water flow ends up in the ocean without providing benefits for wildlife or human society. Conversely, in dry periods, without water being stored in the uplands, the streams and rivers dry up more readily, causing water shortages for wildlife and humans.
Drainage also causes soil erosion due to fast flowing water in the grips. This contaminates water purity which can severely impact wildlife, for example fish eggs, and can cause problems for human water supplies.
Peat, as dead vegetation, is storing carbon. It depends on being permanently waterlogged and anaerobic to maintain this stable state. With drainage, the peat dries out and the carbon is exposed to the air, allowing it to oxidize. As carbon dioxide, it escapes into the atmosphere as a greenhouse gas.
Despite their artificial origins as a result of deforestation, blanket bogs are now valued for their wildlife interest and other benefits.
RESTORATION
Bog restoration at Bwlch Corog is being achieved by reversing the drainage to restore the natural hydrology of the peatland. The drainage grips were blocked during 2019 using a series of peat dams spaced every 10 metres along the grips. The peat for the dams is taken from around the grip, so that a hollow is dug above the dam: as the flow is stopped a pond is formed. We now have over a thousand small ponds across the site, creating new habitat for wildlife. Over time, the stagnant water will become colonised by plants and the dead plant material will fill up the ponds and ditches.
In the meantime, the natural hydrology is restored to the peatland with immediate effect. The ground is clearly wetter or indeed waterlogged each side of the grips. The effect on vegetation will be to kill off Molinia which will gradually be replaced by sphagnum mosses. A more immediate effect is the colonisation of heathland plants in the exposed peat on the pond edges.
There are a few significant patches of sphagnum moss across the moorland. It has also survived in small clumps hidden amongst the Molinia. Some bog restoration projects have introduced sphagnum moss to accelerate the restoration process. This is a possibility at Cambrian Wildwood; we are also willing to to take a long view and minimise further intervention. Having removed the unnatural drainage of the site, it will be interesting to see how the vegetation develops.
For more detail on the grip blocking work, see our Bog Restoration news post
For more information on the vegetation at Bwlch Corog see:
Vegetation survey of Bwlch Corog, Ceredigion 2005 (part 1)
Vegetation survey of Bwlch Corog, Ceredigion 2005 (part 2)
Bwlch Corog Vegetation Assessment Sep 2017